There are many chemical applications, particularly analytical applications such as those involving the use of microfluidic analytical devices known as chips and the like, which involve the use of pressurized liquid solvents, aqueous reactants and the like delivered to a microfluidic reaction system or chip to perform a desired analysis. Microanalyses generally use small amounts of reagents and thus allows the use of costly reagents and techniques not otherwise economically feasible. Examples of such microfluidic systems are increasingly found and used for hematology, cytometry, DNA analysis and other such analysis systems. The goals of microfluidic systems include reduced reagent use, reduced size of the sample needed to generate a signal for analysis, reduced footprint of the instrument, improved sample throughput and reduced cost per analysis. To achieve these goals, microfluidics systems typically utilize fluid passageways ranging from 1 to 100 microns in diameter, and which may be shaped to fit a desired manufacturing technique. It is commonly observed in such small devices containing small diameter fluid pathways and correspondingly small sensors that air bubbles can be trapped within the fluidics thereby occluding a region on a sensor or blocking a portion of the flow path.
Various approaches have proven successful in preventing the formation of bubbles, including the use of degassing devices such as those shown in U.S. Pat. Nos. 5,340,384, 7,144,443, 6,949,132 and 7,713,331, the contents of which being incorporated herein by reference. While each of these devices may at least optionally employ a vacuum to degas the fluid passing therethrough, the goals of the microfluidic system designer may be such that the use of a vacuum pump is undesirable. Additionally, the design goals of the same designer may be such that the use of a pumping system which directly contacts the reagent pathway is also undesirable. Microfluidic analysis systems are typically designed with pressurized containers containing the reagents required for the analysis, although a vacuum may also be used to draw fluids from their respective containers. Early examples of such pressurized systems are contained in U.S. Pat. Nos. 4,994,180 and 4,598,049 wherein a pressurized gas is used to deliver solvent to a liquid chromatographic pumping system and reagents to a DNA synthesizer, respectively.
In the instance wherein a gas pressurized system is used to provide the motive force to enable fluidic flow to a microfluidic system or chip, the fluid generally becomes supersaturated by the gas being used to pressurize the fluid. Supersaturation may be treated through costly means such as multiple layer bags with a near-zero permeability wherein the gas pressurization is provided external to the bag, or by mechanical pressure applied to the fluid. Gas-impermeable bags are generally manufactured from multiple layers of various plastics and may also involve the use of metal-coated layers to limit gas permeation to acceptable levels. Such reagent bags are single-use, they contribute to the cost of every analysis, and may be limited to short exposures to pressurization by gas before the contents become supersaturated with the pressurization gas. Mechanical pressurized systems are not widely used due to sealing considerations between the driving means and the fluid. As described in U.S. Pat. No. 4,133,767, outgassing of atmospheric pressurized liquids was found to be limited if such solvents were sparged of dissolved atmosphere using helium as a sparging gas. One could speculate that due to the low solubility of helium in fluids such as common solvents and water, simple overpressurization of reagents by the use of helium may be used. However, other concerns regarding the use of helium arise. First, helium quickly supersaturates the pressurized fluid, and although of limited solubility, helium nevertheless forms bubbles when the fluid is exposed to a lower pressure. Secondly, helium is of limited availability, expensive and requires bulky containers. Thirdly, many of the desired chemical reactions in microfluidic systems are accelerated or inhibited by the exclusion of oxygen that may be brought about by helium or other inert gas pressurization. The use of a membrane to release the supersaturation gas to the atmosphere and to achieve near-equilibrium concentrations of dissolved atmosphere in the pressurized fluid is advantageous in that it retains the driving force provided by the gas pressurization of the reagents yet removes much of the supersaturation of the fluid by the pressurization gas.
Conventional degassing systems do not well accommodate the transport and condensation of the liquid vapors permeating through the membrane into the space between the membrane and the outer impermeable shell (the degassing chamber). Instead, the transport of vapors out from the degassing chamber is performed by the application of a vacuum and/or a sweep gas. Without active removal of the vapors accumulating in the degassing chamber, temperature changes may cause a build-up of condensate within the degassing chamber. Condensate build-up can block the free flow of dissolved gasses through the membrane into the degassing chamber by occluding part or all of the membrane surface. In the extreme, a liquid-gas contactor can become “flooded” and cease functioning as a gas-permeable barrier altogether. Condensate build-up can also cause mechanical interference with the action of the vacuum pump. Condensate is typically removed from conventional systems by a purge gas port or by allowing air to enter the fluid side of the membrane to provide sufficient air flux to purge the permeate side of the membrane. The practice of condensate removal requires some external means of applying gas flow to the degassing chamber.
While conventional degassing systems employ mechanisms and/or processes through which condensate may be removed from the degassing chamber, certain degassing applications do not employ the mechanisms of conventional systems necessary to purge and/or dilute condensate vapors in the degassing chamber. For example, certain degassing applications do not employ a pumping system, either in the form of a vacuum pump or a sweep fluid pump. Moreover, a mechanism for introducing external gas flow to the permeate side of the membrane may not be available or desired. In such circumstances, a mechanism is needed to remove the permeation vapors from the degassing chamber. Preferably, such a mechanism does not substantially add to the cost or complexity of typical shell and tube degassers.